Paper treating resin



Patented June 8, 1954 PAPER TREATING RESIN William C. Dearing, Troy, N. Y., assignor, by mesne assignments, to Allied Chemical & Dye Corporation, New York, N. Y., a corporation of New York 1 No Drawing. Application January 26, 1950,

Serial No. 140,745

position of improved stability and watersolubility that is capable of imparting greater wet strength to paper than the synthetic resins heretofore used for the impregnation of paper.

When used for imparting Wet strength to paper, a synthetic resin is usually incorporated at the Wet end of the paper making process, for example, in the beater or at the head box. A synthetic resin that is incorporated at thev wet end of the paper making process must be capable of dilution without precipitation of the resin, and must have an affinity for the paper fibers so that a reasonably large proportion of the resin deposits on the paper fibers and so that an unr asonably large proportion of the resin is not lost in the waste water.

The principal object of the invention is to provide a thermosetting composition of improved stability and water solubility comprising a syn-' thetic resin that has the special properties necessary to permit it to be incorporated at the wet end of the paper making process and also aim-- parts substantially higher wet strength per unit of cost than the resins heretofore used for the impregnation of paper. More specific objects and advantages are apparent from the description, which illustrates and discloses but is not intended to limit the scope of the invention.

A product of the reaction of formaldehyde, urea and a hydroXy-substituted carboxylic acid having not more than four carbon atoms per carboxy group imparts greater wet strength to paper than the synthetic resins heretofor used, but such a reaction product, when made from ordinary commercial formaldehyde, has very low stability so that the water solubility of the reaction product rapidly decreases during storage or shipment. Thus, although the reaction product when freshly prepared is capable of imparting superior wet strength to paper, its stability is so low that it is not usable after it has been shipped from the manufacturer to the user and, therefore, it is not marketable in accordance with ordinary commercial practice.

The present invention is based upon the discovery that a composition comprising a polyhydric alcohol having not more than two carbon atoms per hydroxy group, and a product of the reaction of formaldehyde, urea and a hydroxysubstituted aliphatic carboxylic acid whose molecule has not more than eight carbon atoms and has not more than four carbon atoms per carboxy group not only imparts superior wet strength to paper but also is stable and has a 2 Claims. (01. 260-40 solubility in water that does not decrease materially during ordinary storage or shipment and also is higher than the water solubility would be if the glycol were omitted.

In the preparation of a thermosetting composition embodying the invention the formaldehyde used may be wholly or partially in the form of one of its polymers such as paraformaldehyde, but preferably is in the form of an aqueous solution (as hereinbefore described). The hydroxy-substituted carboxylic acid may be any substance whose molecule consists of a substituted straight or branched hydrocarbon chain having at least one carboxy group and at least one. hydroxy group, each attached to a primary, secondary, or tertiary carbon atom, andhaving no substituents other than carboxy and hydroxy groups, said molecule having not more than four carbon atoms per carboxy group, i. e., having from one to three additional carbon atoms per carboxy group and having not more than a total of eight carbon atoms. It is preferred that the substituents be attached to primary or secondary carbon atoms. Such substances include glycolic acid, lactic acid, hydroxyglutaric acid, hydroxybutyric acid, malic acid, and tartaric acid. For the sake of brevity the term hydroXy-substituted carboxylic acid is used hereinafter to designate such substances. The polyhydric alcohol (for the sake of brevity, hereinafter referred to as a glycol) may be any polyhydric alkane or linear ether thereof, which is otherwise unsubstituted, and in which there are not more than two carbon atoms per hydroxy group. Such substances include glycerine; ethylene glycol, diethylene glycol, a-propylene glycol, trimethylene glycol, erythritol, pentaerythritol, dipentaerythritol, adonitol, arabitcl, xylitol, mannitol, sorbitol, dulcitol, and iditol. The term glycol is used hereinafter to designate such substances or mixtures thereof.

It is preferred that a thermosetting composition embodying the invention comprise a glycol, and a product of the reaction of formaldehyde, urea and a hydroxy-substituted carboxylic acid in which carboXy radicals are neutralized with an alkali metal base (for reasons hereinafter discussed). The term alkali metal base is used herein to mean any alkali metal compound that gives an alkaline solution, the preferred alkali metals being sodium and potassium. Such alkali metal bases include sodium hydroxide, potassium hydroxide, sodium alcoholates such as so dium methoxide, sodium ethoxide, and sodium beta-methoxyethoxide (obtained simply by dissolving metallic sodium in methanol, ethanol or beta-methoxy ethanol, respectively), potassium carbonate, sodium tetraborate, potassium tetraborate, and sodium bicarbonate. The preferred alkali metal bases are sodium hydroxide and potassium hydroxide. Sodium hydroxide is more desirable from the standpoint of economy.

It is preferred that the hydroxy-substituted carboxylic acid be glycolic acid. It is preferred also that the glycol contain only one carbon atom per hydroxy group (e. g., as in glycerinefiet'hyl'ene glycol, erythritol) and most desirable that it be glycerine. composition of th invention be in'the form of an aqueous solution, as hereinafter described.

The glycol is believed to react to some extent with the other components of' the resin. Although the glycol may be mixed with the resin after it is prepared, and its mere presence in the water solution will make the resin more soluble and more stable than a resincomprising a similar reaction product whose aqueoussolution contains no glycol, even betterlresults are obtained when the glycol isieactedwith the resin. It is preferable, therefore, 'toad'd' .the glycol during the resin preparation because reaction is then assured. A most desirable composition of the invention is a thermosetting composition comprising, in aqueous solution, a'prodnot of the'reaction of glycerine, formaldehyde, urea, and glycolic acid, inwhich the glycolic acid is neutralized with sodium hydroxide.

In the practice of the invention not more than about 2.2 mols of formaldehyde should be used per mol of urea, and it is preferable to use not more than about 2.15 mols per mol of 'urea. Not less than about 1.8 mols of formaldehyde should be used per mol of urea and it is preferable to use not less than about 1.9 mols per mol of urea. It is most desirable to use about 2.12 mols of formaldehyde per mol of urea.

Not less than .05 mol of the hydroxy-substituted carboxylic acid should be used. per mol of urea. It is preferable that the amount of hydroxysubstituted carboxylic acid used be not less than about .15 mol per mol of urea. It is preferable to use not more than about .25 mol of the hydroxy-substituted carboxylic acid per mol of urea. It is most desirable to use about .2- mol'of the hydroxy substituted carboxylic acid per mol of urea. Increasing the amount of the hydroxysubstituted'carboxylic acid to as much as .5 mol per mol of urea tends to decrease the degree of wet strength imparted by thermosetting compositions embodying the invention.

The amount of glycol used in'the'practice of the present invention is preferably not greater than about 25 per cent andnot less'than about '10 per cent of the weight of urea employed, although amounts as large as 30 per cent or as small as 5 per cent may be used. It is most desirable to use about 20 per cent of the glycol based on the weight of urea. (The term per cent as used herein to refer to quantities of material means per cent by weight unless otherwise qualified.)

As hereinbefore stated, a thermosetting composition embodying the invention is capable of imparting superior wet strength to paper. Wetstrengthened paper is extremely useful in all paper products which may come in contact with water or moisture during use, for example toweling, food product wrappers, bag and wrapping paper, and map and blue print paper.

A synthetic resin for imparting wet strength to paper is desirably incorporated atthe wet .This more convenient and less expensive method of applying the resin to the paper results in a more porous paper which is not coated with a sealing. Since in the production of wet strength 'apaper'rthe mixture at the point of resin addi- It is desirable that'the thermosetting tion ordinarily comprises a very dilute suspension of' pulp in water'i'less than two per cent) and a synthetic"resinused for imparting wet strength is usually present in this suspension in a concentration of about one to two per cent of the 'pulp' concentration, the resin must be capable or dilution without precipitation. Such a resin should be a thermosetting composition so that it can be added in its *soluble stage to disperse and dissolve throughout the paper pulp suspension at the wet end or the paper making process beforeithe'paper is'm'ade, and then can be converted to a thermoset resin on the paper fibers by heating duringjdrying or ageing during'ston age.

Although theoretically ordinary urea' formaldehyde resins are soluble in theconcentrationrange requiredduringwet end addition, in actual practice these thermosettin'g resins form curds when incorporated at the wet end under the "slightly acid'conditions used, before they have had time to be'dispersed and dissolved in water. The

curds cannot be'readily' dissolved and stick to the apparatus so that much of'th'e resin is deposited on the apparatus or lost in the waste water. (The formation of curds necessitates frequent cleaning of the apparatus and leads to serious difficulties, because under the acid conditions used the curds, after depositing on the equipment'are converted to the insoluble state.) The small amount of-Jresin that does cling to the paper fibers does not coat them uniformly.

Such a resin makes the paper non-absorbent and is undesirable for use in imparting wet strength to paper.

' Since urea-formaldehyde resins when used as paper treating resins to impart a high degree of wet strength must besufilciently water soluble to dissolve so rapidly that curds do not form it is necessary to use a modifying agent which gives a high degree of water solubility to such resins. Ithas been found that thisis accomplished by the presence-of an ionic charge attached to the resin molecule, which givesit an electrical charge when insolution. The hydroxy-substituted carboxylic acid, or a salt thereof, used in the practice of the present invention becomes part of the resin through its .hydroxy substituent. The ionization in solution of the substituted carboxylic'acid which is thus a part of the resin molecule imparts a negative electric charge to the molecule. Such a soluble anionic resin imparts wet strength to paper when it is converted to a thermoset resin on the paper fibers by heating during drying or ageing during storage. It is believed that the presence of the solubilizing groups in the resin molecules. on the fiber also impartsgreater absorbency tothe paper since the soluble resin is less water repellent and hence facilitates passage of water through thethin layer of resin on the fiber. The known sodium bisulfite modified resins do not have as good water solubility and do .notzimpart as high a degree of wet strength topaper per pound of resin used as a synthetic Sodium bisulfite may be used along with the hydroxy-substituted carboxylic acid in the practice of the invention. Such a modifying agent should not be present in a concentration greater than about 30 per cent of the hydroxy-substituted carboxylic acid, since its use in larger concentrations tends to give a compound of the invention which imparts lower wet strength to paper than the compounds formed by using only the hydroxy-substituted carboxylic acid.

The quantity of a thermosetting resin embodying the invention used to treat paper pulp' is within the range ordinarily used in the art of making wet-strengthened paper and varies with the degree of wet strength desired. For example, the minimum desirable concentration of resin is the smallest concentration that gives an appreciable increase in the wet strength of the paper (i. e., about 0.1 per cent resin based on the weight of dry pulp). The maximum desirable concentration of resin (i. e., about per cent based on the weight of dry pulp) is that above which the wet strength imparted to paper is not increased enough to make a larger concentration of resin economical for most applications. A concentration between about 0.25 and about 5 per cent ordinarily gives considerable wet strength economically and emciently. The paper pulp used may be of any type such as bleached or unbleached sulphite, kraft or ground wood pulp. The resin may be added to the pulp at the wet end of the paper machine as prepared (i. e., as an approximately 50 per cent water solution) or, if desired, it can be diluted to about per cent solids, preferably with water at a temperature above 50' degrees F. The liquid resin is infinitely dilutable with water and will, therefore, be easily and uniformly dispersed throughout the stock in the beater, machine chest, fan pump, head box or any other desired point at the wet end of the paper machine, preferably after all fiber refinement. It is desirable to add a catalyst such as alum or aluminum chloride about five minutes before the resin addition. The resin and catalyst may stand on the pulp for a few hours before the mixture is used for making paper. Such contact time should not, however, approach as long a period as 24 hours since a portion of the wet strength is lost under such conditions. The pH of the mixture should be adjusted within a range of 4.0 to 5.5, the pH being lowered to this range using for example, dilute sulfuric acid. It is most desirable that the pH of the mixture be about 4.5.

The wet strength imparted by a thermosetting resin of the invention is aflected when the pH of the pulp suspension prior to the addition of the resin is varied. The hardness in the water used to dilute the pulp suspension is another factor which affects wet strength.

In the production of a thermosetting composition embodying the invention the reaction medium consists of water, usually that present in a commercial formaldehyde solution, which ordinarily is an aqueous solution consisting of 37 per cent formaldehyde and 63 per cent water. The only other source of water in the reaction mixture is that present, if any, in the hydroxysubstituted carboxylic acid, which may contain about per cent of water. Further dilution during the reaction is undesirable, since it results in resins having decreased stability.

Resins embodying the invention reacted at the concentration of solids achieved using an ordinary commercial formaldehyde solution and the molar proportions of reactants hereinbefore described have greater stability and water solubility than resins heretofore known for imparting wet strength made from ordinary commercial formaldehyde. The'degree of water solubility of resins used for imparting wet strength is dependent upon the number of solubilizing groups which become part of the resin molecule. "since using an amount of hydroxy-substituted carboxylic acid above the desirable limit hereinbefore described tends to decrease the wetstrengthening properties of compositions embodying the invention, it is not desirable to increase the amount of hydroxy-substituted carboxylic acid above such limit to improve the water solubility of the resins. It has been discovered that less dilution during the reaction, 1. e., reacting at a higher solids concentration than is achieved using ordinary commercial formaldehyde, results in resins having a higher degree 01" water solubility, since there is then a tendency for more of the solubilizing groups to become part of the resin molecule. Such resins, when diluted after preparation, also have greater stability. A resin embodying the invention when reacted at a higher solids concentration than is achieved using ordinary commercial formaldehyde has a greater degree of Water solubility (and stability, after dilution) than a resin reacted at a similar increased solids concentration but containing no glycol.

The solids concentration of the reaction mixture may be increased by using the neutralized substituted carboxylic acid in the form of its crystalline salt. For example, a resin embodying the invention prepared with crystalline sodium glycolate is more soluble and has better stability than a similar resin prepared with an equivalent amount of a solution of 70 per cent glycolic acid neutralized with flake caustic. Even greater improvement in the water solubility of resins embodying the invention is obtained if the concentration of solids in the reaction mixture is increased by using a formaldehyde solution containing a greater concentration of formaldehyde than 37 per cent. It is desirable to use an aqueous solution containing a concentration of to per cent of formaldehyde, since use of such" a concentrated formaldehyde solution gives substantially improved properties economically and efficiently. If desired, paraformaldehyde may be dissolved in a 3'7 per cent aqueous formaldehyde solution to raise the concentration of formaldehyde to 45 or 50 per cent.

A commercial formaldehyde solution is in itself highly acid so that the pH of the reaction mixture containing also the hydroxy-substituted carboxylic acid is quite low. Since the reaction of formaldehyde and urea tends to go too rapidly in astrongly acid'solution so that the product els, and since the reaction does not proceed :itis desirable that the pHnf thereaction :mixture be. adjusted within the range 5.2 to 5.6. Varying tneireaction pH-between 5.2'and 5.6 and varying the reaction temperature has no substantial effect in general on the properties of resins reacted to the same degree of condensation. Since, however, undesirable by-products form within this pHirange at temperaturesbelow around 95 degrees C.'it is desirable to lower thepH to within the range from 5.2 to 5.6 only after the temperature of the reaction mixture reaches about 95 degrees C., to-avoid turbidity in the final product. (Although formation of an insoluble by-product has no effect on the wet-strengthened paper made from filtered resin, the presence of a pre- "cipitate makes colorimetricpH control during the resin preparation verydifficult.) It is, therefore, most desirable to raise the initial pH with a'base within a range of about 6.5 to 7.0 and to maintain the pilot the mixture in approximately this. range until wthe temperature is about 95 degrees C.

It is preferable to usea strong base such as an alka'li metal base (ashereinbefore defined). Such-a base willalso'neutralize part-or all of the hydroxy-substituted carboxylic acid. At the time ofreactlon it is preferred "that from to 100 mol per cent of the hydroxy-substituted carboxylic acid taking partin the reaction be in theform of a metal :salt that is water soluble .and does not interfere inthe reaction. The product of such a reaction is a composition embodying the invention because the salt is largely ionized .i-n theaqueous-reaction medium, and some of the anions produced by'such ionization combine with hydrogenzions to form molecules of the carboxylic acid .50 that'some of the-molecules taking part in the reactionare-molecules of the free carbox'ylic acid. H the watenpresent during the reaction is removed by dryingthe reaction product, and if the cations from themetalbase are present in an amount sufiicient ,tocombine with all of the carboxy groups in the molecules of the reaction product, the ionization which toolr'place when the-alkali metal salt dissolved in the aqueous reaction medium isreversed :during the drying of thereaction product and allthecarboxy groups in the driedreaction-product are neutralized by the metal base.

In the practice of the inventionthe resin components may be mixed any desiredorder. It is usually-desirable to neutralize the 'carboxylic acid first. The formaldehyde solution and the glycol may then be added and the mixture heated for about two hours, prior to the addition of urea and suflicient base -.to raise the pH within the range from 6.5 to 7.0. Alternatively the urea and formaldehyde solution may be adjusted to the proper pH and reacted 'for about minutes prior to the addition of the neutralized hydroxysubstituted carboxylicacid and the glycol. The glycolmay be added at some period during the reactiorc It is preferable, however, to add it before-starting thereaction. Most desirably the formaldehyde solution, urea and glycol are'added successively to the neutralized hydroxy-substituted carboxylic acid, and additional base is then added to :raise the pH of the mixture within the range from 6.5 to 7.0. The mixturethen either may beheated for one hour at degrees C. or may be allowed to stand overnight.

It is-believed-that during this preliminary reactlonstage one molecule of formaldehyde com bineswlth one molecule of water to (form methyl- Resins ene glycol, .and it is believed to be themethylene glycol that actually takes part in the reaction with a substance such as aurea, so that a methylol group is formed by the condensaion of'a'methylene glycol molecule with an NHz group in "the urea molecule; one molecule of water being-eliminated in such condensation:

Thermosetting compositions of the invention having equally as good properties maybe obtained without this much preliminary reaction. It is necessary merely to allow enough time for'the reactants to dissolve before continuing thereaction as hereinafter described.

In general, more highly condensed resins impart better wet strength to paper. However; the increase in Wet strength imparted by a givenresin may be inappreciable beyond a certain'degrceof condensation (viscosity) and since increased condensation tends to decrease both the stability and the water solubility of resins embodying-the invention, the reaction should be terminated when that viscosity has been reached. The viscosity of resins reacted to the same degree of condensation will, of course, difi-er in accordance-w-i-ththe solids concentration of the reaction mixtures. embodying the invention in which the proportions of resin reactants are the desirable proportions hereinbeiore given and in-which-the concentration of formaldehyde in aqueous solution is approximately (a) 37 per cent, (1)) 50 per cent or (c) 45 per centare reacted'toanesirable degree of condensation as follows:

(a) The pH of the reaction mixtureis initially adjusted to 6.5-7.0, the mixture is. heated to a temperature of about '95 degrees C. before'lower" ingthe pH to 5.2-5.6, and the reaction is continued at thistemperature until the viscosity :of the resin solution is about 27seconds (measured by Ford Cup, 3% inch opening, at 25 :degrees 0.). The reaction usually requires about eight to nine hours when the pH is about 5.6, or from "two to six hours when the pH is'within the range 5.3 to 5.45. The mixture is then cooled to .60 degrees C. and held at this temperature until-the viscosity :of the solution is 32 to 34 seconds (Ford Cup), the pH being adjusted to 5.6 at'the'beginning of this period. This latterreaction' stage usually requires from one to three hours.

'(b) The pH of the reaction mixture isinitially adjusted to 6.5-7.0, and the mixture is heated to atemperature of 95 degrees C.,'two subsequent pH adjustments being made, one to" 7.5 at 50 degrees C. and the other to 7.2 at degrees C. to avoid precipitate formation. It is desirable "that the temperature rise from 65 degrees C. to degrees C. in less than 45 minutes. The mixture is held at 95 degrees C.'for five minutes before lowering the pH to about 5.6, and the solution is held at this temperature and pH until the viscosity of the resin solution is J (measured by Gardner Holdt bubble viscometer standard method, the Ford Cup viscosity method becoming increasingly inaccurate at the much higher-viscosity encountered using the more concentrated reaction mixture). The mixture is then rapidly cooled to 60 degrees C. (about 10 minutes cooling time) and held at this temperature until-the viscosity of the solution is V-W (Gardner- Holdt) (c) The resinis prepared :by the procedurekdescribed in (b) except that the viscosity of'the solution after the 95 degree stage should be .H-I, and the final viscosity after the 60 degree stage should be U"-V (Gardner-Holdt).

Using a two-stage reaction (that is, heating first at 95 degrees C. to a certain viscosity and then reaching the final viscosity by reacting at 60 degrees C.) ordinarily makes the reaction more controllable and gives more reproducible results than a one-stage reaction carried to the same final viscosity at 95 degrees 0., although there is no essential difference in the properties of the final product. Usually, the total reaction time must be at least three to four hours in order to allow sufficient time for the viscosity measurements and the temperature and pH adjustments that are necessary for safe control.

The wet strengthening properties of a resin of the invention may be appreciably increased by aging the resin for a period of time at room temperature or even lower temperatures. There is also a gradual improvement in solubility on aging. The type of condensation that takes place on aging at room temperature seems to diifer from that which takes place during the actual reaction carried out at higher temperatures. During the reaction the cloud temperature (i. e., the temperature above which a drop of resin solution dissolves in a given amount of water leaving a substantially clear solution) gradually increases to a certain point where it tends to level off. On aging, resin samples show a gradual decrease in cloud temperature up to the point of gelling, at which there is a sharp increase in cloud temperature.

. It is desirable to neutralize the liquid resin with a base such as sodium hydroxide to a pH of at least 7.0. Preferably the pH is adjusted to the range 7.8 to 8.0, for maximum stability. The pH of the resin solution decreases on standing, the decrease being more rapid at higher temperatures. The decreasing pH of the resin solution is probably at least partially due to the formation of formic acid through air oxidation (and Cannizzaro type reaction) of the free formaldehyde usually present. Therefore, abuffer such as sodium bicarbonate, sodium borate, or borax is usually added to retard the rate of pH drop. Although use of 0.5 per cent of borax (based on the weight of solids in the resin), or 0.5 per cent of sodium bicarbonate, or 0.2 to 0.3 per cent of sodium borate gives good results, it is more effective to add a buffer mixture containing 70 per cent boric acid and 30 per cent borax (in a ready prepared per cent water solution) to the extent of 1 to 3 per cent (based on the weight of resin solids). Resins embodying the invention which are prepared using a more concentrated reaction mixture are most desirably adjusted to a pH of 8.0 and buffered with 2 per cent (based on the Weight of solids in the resin) of a buffer mixture containing 65 per cent boric acid and 35 per cent borax.

Resins reacted according to the procedure described in'(a) ordinarily contain about 48 to 50 per cent solids, and those reacted according to procedures (6) and (c) ordinarily contain about 51 to 55. per cent solids. The good stability of a resin embodying the invention can be further improved by diluting the resin so that the solids concentration is about 45 per cent. Although further dilution may improve the stability even m re. this concentration seems most advantageous for shipping and storing the-resin.

'The magnitude of the improvement in wet strength imparted either by a thermosetting resin embodying the present invention or by a resin which is similar in every respect except that no glycol is used, over that imparted by the resins previously used for imparting wet strength, may be demonstrated by tests carried out as follows. The results of these tests are shown in Table 1.

Anhydrous sodium bisulfite (167 grams) is dissolved in methanol-free formalin (1960 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water). Urea (660 grams) and sodium acetate (4.5 grams) are dissolved in this solution in a 3 liter three-necked flask fitted as described above. The solution is allowed to stand overnight at room temperature. The solution is then heated on a boiling water bath for about eight and one-half hours at a temperature ranging from 94.5 degrees C. to 97.5 degrees C." (The buffer action of the sodium acetate keeps the pH at approximately 5.6 during the latter part of this heating period.) The mixture is allowed to stand overnight and is then held at a temperature of 60 degrees C. and at a pH of 5.6 for five hours before cooling to room temperature. This resin is resin A in Table 1, a resin of the type heretofore used for imparting wet strength (the control resin) v A hydroxy-substituted carboxylic acid (87.5 grams of glycolic acid in 37.5 cc. of water) is mixed with fiake caustic (50 grams) and to this mixture in a 1 liter three-necked flask fitted with a thermometer, reflux condenser, stirring rod and oil seals is added urea (240 grams), a glycol (48 grams of glycerine) and methanol-free formalin (687 grams of an aqueous solution consisting of 37 per cent formaldehyde and 63 per cent water). The mixture is held at temperatures ranging from 94 to 98 degrees C. at a pH of 5.4 for three hours. The solution is then cooled to 60 degrees C. and held at this temperature for one hour before cooling to room temperature and neutralizing to a pH of 7.0 with sodium hydroxide. This resin is resin B in Table 1.

Resin C in Table 1 is prepared by the procedure used for resin B except that no glycerine is used.

Resins A, B and C are used to'treat paper prepared and tested by the following procedure:

. Pulp (containing the equivalent of 360 grams of oven-dried pulp) is soaked in water (10 liters) overnight. The soaked pulp is then agitated for 10 minutes with a Lightnin mixer (a high-speed motor-driven stirrer). The agitated suspension is then placed in a Valley beater (a standard beater designed for laboratory use) and enough water is added to bring the total volume of water to 23 liters (measured at a temperature of 25 degrees 0.). The heater is run for five minutes (slush period) before a load (4500 grams) is placed on the lever arm which applies a force to the beater roll. Samples are withdrawn at various intervals during the beating to measure the rate at which water passes through the pulp (freeness) as Schopper freeness. The freeness of an 800 ml. sample after twenty minutes beating time is about 700 and that for an 800 m1. sample after about thirty minutes beating time is 550. The beating is terminated after thirty minutes.

The beaten pulp is diluted to such an extent that a volume of approximately 800 ml. gives a dry sheet weighing 2.0 grams. The pH is adjusted" to 6.5 by the addition ofsulfuric acid. Alum (a minum sulfate in about cc. of water) is added.

with stirringto the beaten pulp suspension which is then 1ready iorthegadditionof. the. resin iorirnparting wet strength. The beaten. .pulp;.suspen.- SiOII riS. allowed to stand for fiveminutes before a resin-e for imparting wet strength (an amount of a; resin solution sufiicient to give- Bpercent based-on the weight ,of' the dryjpulp) is added. A volume-ofstock large. enough: to given sheet of thedesired 2.0 grams weight (800 ml.) .-is placed in the: sheetmachine. and .dilutedto a total-rol umeof 10.7. liters, and the pH. is. adjusted-to 4.5 by addition of sulfuric acid; The handsheetcis. madecwithin five minutes after'the addition of the. resinqandthe operation is repeated four; times. without; delay; to make; four. more sheets...

Thefhandsheets. aromadeaccording. to Instituteiof Paper Chemistry-Tentative Method 411'- B-Valley.- The. sheets are;pressed separately between six blotters under apressure of 100 pounds for t-Wominutes- Each sheet is placed. on the drier; whilestill. in contact with one blotter .(sheet against the metal). and driedgfoi" fiveminutesat 250:.degrees The sheets are. conditioned for" a minimum of eight hours in a room at. a temperature .of 75 degreesE, and at 78 percent relative humidity before testing.

Wet strength measurements are made; on the handsheets with a Mullen tester whichimeasures the, burstingpressure, expressed. as points. (approximately ounds .p'ercsquare inchlfor a stand-:

ardized circular area; Bursting strength of the paper; is, given herein as a burst factor, that is, points per 100' pounds of basis weight (basis weight is tnesweignt of 500.-sheets of the paper, inches by inches) Mullen .Wet burst values are 01113831166. on paper samples: :wet, withwater froma-i brush (equivalent to. about .a;ten= second soakof the; paper samples) TABLE 1 Mullen Wet Burst Resin A Resin Bnn; l Resin C It may: be readily seen from Table '1. that'both resins; B and C impart wetstrength. higher than that impartedby resin A and that there is es-.

78 degrees F. Othersamples of. resins B and C adjusted. to the same pH are held at a tempera? tureeofw 120 degrees F. Stabilityof these samples;-

is indicated by thenumber .ot days'the samples remainat these temperatures without gelling;

the results being .recorded in Table. 2;

TABLE" 2 78 degrees 120 degrees F., days .'-F., days ReslllrBrr. 210 10 Resin 0 89 4% The stabilityof a resin-'"containingraglycol- (resin B) is more than twice as great at either temperature than the. stability of a resin (resin C) prepared-in the samemannerzas resin-2B ex. cept that no glycol 'isused.

As hereinbeforedemonstrated; notonly. are res--.

ins embodying thepresent inventioncapableof impartinggreaterrwet strength to paperttharr thesynthetic resins heretofore used for thafimpregnation of paper; .butzthey. also. possess 1111- proved stabilityover resins which may impart equally as high wet strength. .iThat.resinssenri bodying the invention have improved wateri'solu bility in addition: to improved stability maynbe demonstrated by tests'carriedoutras follows: (The results of these tests. are shown'in TableB.) v

A hydroxy-substituted carboxylic acid:v (252 grams of glycolic acid in 108cc. of water) is mixedwithfiake caustic (132 grams);- and the mixture ismaintained for one-half. hour at teni-" peratures rangingfromi'm to degrees'C; To the resulting sodiumiglycolate in a 3+.liter'thr'e'enecked flask fittedtwith a. thermometer-reflux condenser, stirring rod, and. oil sealsis added methanol-free formalin (1981-grams ofasoluti'on' consisting of 3'? per cent iormaldehydeand63 per cent water), urea (690 grams) andaglycol (138 grams of glycerine); Flake caustic-is added'to adjust thepHinto-a rangefromafib 1:071)

The mixture. is. heated .to a temperature of degrees C. and the pH is lowered toapproximately- 5.3 by the addition of glycolicacid; .The heating is continued at temperatures rangingfrom es-to 96 degrees C. until the'viscosity of the solution 1 (Chrome Ford Cup, measured at 25 degrees- &

using a opening). is about 25.5 seconds (attained after about 2 A ihours of heating).- The" mixture is cooled'to a; temperature of 60 degrees C. and the pH is raised to. 5.6 by the addition of sodium hydroxide; The'mixture is the'n held 'at this temperature until: theviscosity" measures about 31.5 seconds. (after about 3 hours at (-60 degrees-0.). The resin is. cooledxto room -temperatureand neutralized-with sodium hydroxide to a pH of 6.98 (glass electrode). -This're'sin isresin K in Table 3.

The control resin, resin D, in Table 3, is" pre-"'- pared like'resin .K exceptzthat no glycerin'e is added. The mixture is.:reacted to a viscosity ob.

25.5 second by; heating for onev hour and40 minutes at temperatures ranging-from to 97 degrees'C anda-viscosity of 31 seconds is-reachedafter the mixture has beencooled .to 60 degrees C. (the-pH being adjustedto 5.6) and held-at this temperature for one-hour.- The resin .is

cooled to room temperature and'neutralized-with sodium hydroxide to a pl-I of 7.05 (glass elec-h trode) The percent solids in the resins (determined: by curmga. weighedsample of the liquid :resin: on a glass plate and weighing the dried sample) is 48.6 in resin K and 48.5 in-resin D. The-viscosity is 32 seconds for resin K and 32.5 seconds" for resin D.

phasesystem forms,- warming thetest tube and" noting the temperature at which the'solution forms aionerphasesystemi' Solubility dataarerecorded in Table 3 interms ofathe =cloudtemperature," i. e.', the temperature above which the one' phase. water solution is formed'at the giyen sol ids concentrations l3 TABLE 3 Cloud Temperature, 0.

Percent Solids 11110. aging-honored Resin K Resin D Resin K Resin D From the table it may be seen that resin K (containin a glycol) is more soluble in water than the control (resin D) at every solids concentration, and that the cloud temperature of resin D is approximately one and one-half times as high as that for resin K at every solids concentration. The cloud temperature of all samples decrease about 3 or 4 degrees during one month of aging.

Th improvement in stability of a resin embodying the invention (resin K) over a resin containing .no glycol (resin D) is demonstrated by the results of tests made on samples of these resins adjusted with sodium hydroxide to various pH values (determined with a Beckman pH meter) and bufiered with a sodium borate-boric acid mixture. Each sample is divided into three portions which are placed at temperatures of 78 degrees F., 90 degrees F. and 120 degrees F., respectively, to obtain the stability results shown in Table 4. Table 5 shows the results of stability tests on samples of th liquid resins diluted with distilled water to various solids concentrations.

TABLE 4 Days (1% buffer) Days (2% bufler) Resin samples Columns 2, 3 and 4 show tests made on 200 gram samples containing 1 per cent buffer, based on weight of resin solids, as follows: Sampl 1: each resin adjusted to pH 8.0 before addition of a buffer mixture consisting of 40 per cent sodium borate and 60 per cent boric acid; sample 2: pH of 8.5, 70 per cent sodium borate, per cent boric acid; sample 3: pH 9.0, 100 per centsodium borate. Columns 5, 6 and 7 show tests made 16 days later on 150 gram samples containing 2 per cent bufier (based on weight of resin solids) as follows: Sample 1: each resin adjusted to pH 7.75 before addition of a buffer mixture consisting of 30 per cent sodium borate and 70 per cent 'boric acid; sample 2: pH 8.0, 40 per cent sodium borate, 60 per cent boric acid; sample 3: pH 8.25, 53 per cent sodium borate, 47 per cent boric acid.

The results in Table 4 indicate that the stability at degrees F. is 3 to 4 timesthestability at 120 degrees F., and that the stability at 78 degrees F. is about 5 to 6 times the ability at 120 degrees F. Resin'K has better stability than resin D at every temperature and with every pH adjustment and buffering combination tested.

TABLE 5 P t Days Stable ercen Resin Solids 78 F. 90 F. 120 I Th stability of both resins K and D increase upon dilution from 49 per cent solids concentration to 40 per cent solids concentration, the stability of resin K being greater than that of resin D at each concentration and temperature tested.

The following examples illustrate the practice of the invention:

Example 1 A resin embodying the invention is prepared by on of the following procedures in which the glycol is not initially reacted with the other resin components.

(a) A hydroxy-substituted carboxylic acid (70 grains of glycolic acid in 30 cc. of water), flake caustic (33 grams), methanol-free formalin (713 grams of a solution consiting or" 37 per cent formaldehyde and 63 per cent water) and urea (240 grams) are mixed in a 1liter three-necked flask equipped with a thermometer, reflux condenser, stirring rod and oil seals. The mixtur is held at a temperature of about 96 degrees C. for two and one-half hours, the pH of the mixture being adjusted to 5.4 with glycolic acid as soon as the temperature is above 90 degrees C. A glycol (48 grams of glycerine) is then added, and the mixture is cooled to 60 degrees C., and held at this temperature for one hour. The pH is raised to 5.6 at the beginning of this period by the addition of flake caustic. The resin is cooled to room temperature, neutralized to a pH of 7.5 with flake caustic, and bufiered by addition or sodium bicarbonate (6 grams).

(b) A hydroxy-substituted carboxylic acid (87.5 grams or glycolic acid in 37.5 cc. of water), flake caustic (50 grams), methanol-fre formalin (687 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) and urea (240 grams) are mixed and heater for three hours at a temperature of degrees C (pl-I 5.4) by the procedure described in (a). The mixture is then cooled to 60 degrees C. and held at this temperature for one-half hour before cooling to room temperature, neutralizing to a pH of 7.0 and buffering by addition of triethanolamine (2.6 grams). A glycol (48 grams of ethylene glycol) is then added.

(0) Other resins for imparting wet strength having good stability and water-solubility, are prepared by a procedure that is the same as the procedure described in (1)) except that the ethylene glycol is replaced with an equivalent amount of one of the following glycols: diethylene glycol, propylene glycol, or pentaerythritol.

1 5 Example 2 A resin embodying the invention is prepared by one of the following procedures in which the glycol is initially reacted with the other resin components.

(a) A hydroxy-substituted carboxylic acid (87.5 grams of glycolic acid in 37.5 cc. of water), flake caustic grams), methanol-free formalin (687 grams of a solution consisting of 37 per cent formaldehyde and 65 per cent later) a glycol (58 grams of diethylene glycol) and urea (246 grams) are mixed in a 1 liter three-necked flask fitted with athermometer, reflux condenser, stirring rod and oil seals. The mixture is reacted for three and one-half hours at a temperature of 95 degrees C., the pH being adjusted with additional glycolic acid to about 5.4 immediately after the temperature reaches 90 degrees C. The solution is then cooled to 60 degrees C. and held for one and one- I half hours at this temperature. After cooling the resin to room temperature, it is neutralized with sodiumhydroxide to a pH of 7.0 and buffered by addition'of triethanolamine (2.6 grams).

This procedure for producing a water soluble, stable resin for imparting wet strength may be carriedout using tartaric acid (150 grams) or lactic acid (90 grams) in'place of the glycolic acid.

(b) A resin having even greater water solubility and stability than the one whose preparation is described in, (a) may be prepared by substituting ethylene glycol ('58gram's) in place of the diethylene glyc ol. The mixture is reacted for four hours at 95 degrees the pH being adjusted to 5.3 with glycolic acid assoon as the temperature is above 90 degrees C. The solution is then cooled to 60 degrees C. and held for three-quarters of an hour at this temperature before cooling to room temperature. The resin is neutralized with sodium hydroxide to a pH of 7 .0 and buffered by addition of triethanolamine (2.6 grams).

(c) A hydroxy-substituted carboxylic acid (252 grams of glycolic acid in 108 cc. of water) is mixed with flake caustic (100.3 grams in 31.7 cc. of water) and the mixture is held at temperatures ranging from 75 to 80 degrees C. (for fifteen minutes) To the resulting sodium glycolate solution in a flask as described in (a) is added a glycol (138 grams of glycerine) methanol-free formalin (1980 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) and urea (GQOgrams). The pH of the mixture is adjusted to approximately 7.4 (Beckman pH meter) by the addition of flake caustic. The mixture is then heated to 90 degrees C. and glycolic acid is added to lower the pH to. about 5.6. The solution is reacted to a viscosity of 32.5 seconds (measured with a Chrome Ford. Cup, inch opening, at 25 degrees C.) by holding at temperatures ranging from 92 to 95 degrees C. for about two and three quarters hours, the pH being about 5.5 during the first half of this period and about 5.4 during the last half. The solution isthen cooled to 60 degrees 0., the pH is raised to 5.6 by the addition of sodium hydroxide, and the solution is held at a temperature of about 61 degrees C. until the viscosity is slightly greater than 33 seconds (about 1 hour at this temperature). The resin is then cooled to room temperature, the pH is adjusted to 7.8 with flake caustic, and sodium bicarbonate (16 grams) is added.

(d) A hydroxy-substituted carboxylic acid (87.5 grams of glyeolic acid in 37.5 cc. of water) is neutralized with sodium hydroxide (50 grams) by the procedure described in (c). To the re sulting sodium glycolate solution is added a glycol (50 grams of pentaerythritol), methanolfree formalin (697 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) and urea (240 grams). The pH is adjusted to 6.8 with additional sodium hydroxide and the mixture is heated to a temperature'of degrees C. The pH is then adjusted, to 5.4 with glycolic acid and the mixture is held for two and one-half hours at about degrees C, The mixture is then cooled to 60 degrees C. and held at this temperature for one-half hour, the pH being raised to 5.6 with sodium hydroxide during the last fifteen minutes. The resin is cooled to room temperature, neutralized with sodium hydroxide to pH 7.1, and buffered by addition of sodium bicarbonate (5 grams).

Example 3,

The following procedures further illustrate the practice of the invention:

(a) A hydroxy-substituted carboxylic acid (90 grams of lactic acid in 90 cc. of water) is neutralized by mixing with sodium hydroxide (50 grams) for fifteen minutes at a temperature maintained between 75 and 80 degrees C. To the resulting sodium lactate solution is added methanol-free formalin (687 grams of a solution consisting of 37 per cent formaldehyde and 63'per cent water) and the mixture is held at 95 degrees C. for two hours, in a 1 liter three-necked flask equipped with a thermometer, reflux condenser, stirring rod and oil seals, the pH being adjusted to 6.0 at the beginning of the period. A glycol (50 grams of glycerine) and urea (240 grams) are then added, the pH is adjusted to 5.4 with lactic acid and the solution is held for five and onehalf hours at a temperature of about 95 degrees C. The resin is then cooled to room temperature and neutralized with sodium hydroxide to a pH of about 7.8.

(b) A hydroxy-substituted carboxylic acid (87.5 grams of glycolic acid in 37.5 cc. of water) flake caustic (50 grams), a glycol (48 grams of glycerine) and methanol-free formalin (252 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) are mixed in a 1 liter three-necked flask equipped with a thermometer, reflux condenser, stirring rod and oil seals, and the mixture is held for two hours at 95 degrees 0., the pH being adjusted to 5.5 at the beginning of this period. Urea (240 grams) is then added, the pH is adjusted to 6.5 with sodium bicarbonate and the mixture is allowed to stand for fifteen minutes before adding methanol-free formalin (435 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water). The heating is then continued for four and threequarters hours at about 95 degrees C., the pH being lowered to 5.4 at the beginning of this heating period by the addition of glycolic acid. The resin solution is then cooled to room temperature and neutralized with sodium hydroxide to a pH of 7.8.

(c) A hydroxy-substituted carboxylic acid ('70 grams of glycolic acid in 30 cc. of water), flake caustic (33 grams), methanol-free formalin (687 grams of a solution consisting of 3'7 per cent formaldehyde and 63 per cent water), a g1ycol (24 grams of glycerine) and urea (240 grams) are mixed and reacted as described in Example 2(a) except that the mixture is held at 96 degrees C. for two hours and the heating period at 60 degrees C. is two hours at a pH of 5.6 (raised with 17 sodium hydroxide). The resin is neutralized with sodium hydroxide to a pH of 7.5 and bufiered by addition of sodium. bicarbonate (6 grams).

(d) A hydroxy-substituted carboxylic acid (70 grams of glycolic acid in 39 cc. of water), flake caustic (33 grams), methanol-free formalin (713 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water), a glycol (48 grams of glycerine) and urea (240 grams) are mixed and heated as described in (c) for two and. one-half hours at 96 degrees C. and one and one-half hours at 60 degrees C. The resin is neutralized with sodium hydroxide to a pH of 7.5 and buffered by addition of sodium bicarbonate grams).

(e) Urea (720 grams) and methanol-free formalin (2064 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) are mixed in a 3 liter three-necked flask fitted with a thermometer, reflux condenser, stirring rod and oil seals. A glycol (144 grams of glycerine) is added, and the pH of the mixture is adjusted with sodium hydroxide within the range 6.5 to 7.0. Crystalline sodium glycolate (174 grams of the monohydrate) is added and the mixture is then heated to a temperature of 97 degrees C. before lowering the pH to 5.3 by the addition of glycolic acid. The heating is continued at temperatures ranging from 93 to 95 degrees C. until the viscosity of the solution is 36.5 seconds (measured by Ford Cup method, attained after about two and three-quarters hours of heating). The solution is cooled to 60 degrees C., the pH is raised to 5.6 with sodium hydroxide, and the solution is maintained at this temperature and pH until the viscosity is 52 seconds (attained after approximately two hours at 60 degrees C.). The resin is cooled to room temperature, the pH is adjusted to 8.5 with sodium hydroxide, and a buffer (0.686 gram of sodium borate mixed with 0.294 gram of boric acid) is added.

(7) A hydroxy-substituted carboxylic acid (375 grams of a solution consisting of 70 per cent glycolic acid and per cent water) is neutralized water, prepared by vacuum distillation of commercial 37 per cent formalin), urea (729 grams) and glycerine (144 grams). The mixture is heated to a temperature of degrees C. before the pH is lowered to 5.5 with glycolic acid. The heating is continued at approximately degrees C. for one hour. The solution is then cooled to 60 degrees C. and held at this temperature for about one hour. The resin is cooled to room temperature and diluted to about 45 per cent solids (with approximately 400 ml. of distilled water), and the pH is adjusted to 7.8 with sodium hydroxide. A buffer (0.164 gram of borax mixed with 0.381 gram of horic acid) is added to the resin solution.

I claim:

1. A paper treating resin of improved stability and solubility in dilute aqueous solution that is capable of imparting superior wet strength to paper, comprising a water-soluble intermediate condensation product of (a) formaldehyde, (b) urea, said formaldehyde and urea being in a proportion of one mol of urea for each 1.9-2.15 mols of formaldehyde, (6) an alkali metal salt of glycolic acid, said alkali metal salt of glycolic acid being in a proportion of 9.15-0.25 mol for each mol of urea, and (d) a polyhydric alcohol of the class consisting of polyhydroxy alkanes and linear ethers thereof whose molecule has from 2-10 atoms, has not more than two carbon atoms per hydroxy group and has no constituents other than hydroxy groups, said polyhydric alcohol being 10-20% by weight of the amount of said urea.

2. A paper treating resin as claimed in claim 1 wherein said polyhydric alcohol is glycerol.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,317,181 DAlelio Apr. 20, 1943 2,322,887 Schwartz June 29, 1943 2,322,888 Schwartz June 29, 1943 2,374,812 Gutkin May 1, 1945 2,389,415 DAlelio Nov. 20, 1945 2,524,111 La Piana Oct. 3, 1950 FOREIGN PATENTS Number Country Date 320,915 Great Britain Oct. 28, 1929 535,745 Germany Oct. 19, 1931 933,891 France Jan. 5, 1948 

1. A PAPER TREATING RESIN OF IMPROVED STABILITY AND SOLUBILITY IN DILUTE AQUEOUS SOLUTION THAT IS CAPABLE OF IMPARTING SUPERIOR WET STRENGTH TO PAPER, COMPRISING A WATER-SOLUBLE INTERMEDIATE CONDENSATION PRODUCT OF (A) FORMALDEHYDE, (D) UREA, SAID FORMALDEHYDE AND UREA BEING IN A PROPORTION OF ONE MOL OF UREA FOR EACH 1.9-2.15 MOLS OF FORMALDEHYDE, (C) AN ALKALI METAL SALT OF GLYCOLIC ACID, SAID ALKALI METAL SALT OF GLYCOLIC ACID BEING IN A PROPORTION OF 0.15-0.25 MOL FOR EACH MOL OF UREA, SAID (D) A POLYHYDRIC ALCOHOL OF THE CLASS CONSISTING OF POLYHYDROXY ALKANES AND LINEAR ETHERS THEREOF WHOSE MOLECULE HAS FROM 2-10 ATOMS, HAS NOT MORE THAN TWO CARBON ATOMS PER HYDROXY GROUP AND HAS NO CONSTITUENTS OTHER THAN HYDROXY GROUP, SAID POLYHYDRIC ALCOHOL BEING 10-20% BY WEIGHT OF THE AMOUNT OF SAID UREA. 